The invention relates to a brush device for an electric machine. The invention also relates to an electric machine using a brush device.
A brush device is known from JP 2002369458 A. In the case of the known brush device, in order to reduce vibrations, it is provided that the brush, which has a rectangular cross-sectional area, is arranged, together with a brush guide element, at a first oblique angle with respect to the longitudinal axis or axis of rotation of a commutator. That face side of the brush which faces toward a commutator lies with its entire cross section against the commutator, so as to form a contact surface. Furthermore, that face side of the brush which faces away from the contact surface has a groove in which a leg of an obliquely arranged spring bears. The spring generates not only a first force component in the direction of the commutator but also a second force component which pushes the brush in the direction of a wall of the brush guide element. If vibrations are generated at the contact surface on the side facing away from said wall of the brush guide element, it is the case even in the presence of relatively low frictional forces that a relatively high tilting moment is generated on the brush, which has an adverse effect in terms of noise generation and vibration reduction, because, owing to the normal manufacturing tolerances between the cross section of the brush and the brush guide element, the brush is guided in the brush guide element with play, and the tilting moment thus leads to an abutment of the brush in the brush guide element.
Furthermore, DE 10 2012 107 327 A1 has disclosed a further brush device in which the brush, which has a rectangular cross section, is arranged within a brush guide element with play in order that, in particular, the longitudinal displaceability of the brush in the brush guide element is ensured. Owing to the guidance with play, a brush device of said type tends to exhibit vibrations. Therefore, in the case of the known brush device, provision is made for the brush guide element and thus also the brush to be inclined at a first oblique angle of 75° to 82° with respect to the longitudinal axis or axis of rotation of a commutator which constitutes the contact element. That face side of the brush which faces away from the contact element (lamella of the commutator) is subjected to the spring force of a spring element, wherein the direction of the spring force runs perpendicular to the longitudinal axis of the contact element and/or to the axis of rotation of the commutator. The spring force of the spring element generates not only the pressure force of the brush against the contact element but also, on that face side of the brush which faces away from the spring element, a force component which pushes the brush in the direction of a side surface of the brush guide element. A disadvantage of said brush device is that, owing to the selected angle of the brush guide element with respect to the axis of rotation of the contact element, with a certain desired pressure force of the brush against the commutator, the pressure force acting on the brush in the direction of the brush guide element is relatively low. Furthermore, owing to the direction of the spring force, which is oriented at least substantially perpendicular to the axis of rotation of the contact element (commutator), there is a tendency for a pressure force to be generated on the brush which is not of uniform magnitude over the length of the brush in the brush guide element, and which in particular decreases in the direction of that side of the brush which faces away from the contact element. As a result, the vibration damping capability of the known brush device is restricted, because in particular, a pressure force which is uniform or which has a certain minimum magnitude over the entire length of the brush is not obtained.